Perf: light worker poll query + tasks(status) index

Worker polls the DB every interval (production 30s) and every
poll ran SELECT * FROM tasks: full rows including result_text
and error_text (up to 4000 chars each) for every task ever,
growing with history. load_tasks now selects only the columns
pick_next actually needs (id/project/prompt/status/depends/
priority/timestamps/executor) and only queued/completed/merged
rows; running/failed rows are neither ready nor completed, so
excluding them cannot change pick_next. get_task still does
SELECT * for the single claimed task. Plus CREATE INDEX IF NOT
EXISTS idx_tasks_status on tasks(status) in both SQLite
init_db and the Postgres schema, so the poll filter stops being
a full table scan on old databases too. Test: load_tasks returns
only queued+completed out of queued/completed/running/failed.

cloud/server.py

@@ -305,6 +305,12 @@ def init_db(db_path):
         )
         migrate_tasks_table(conn)
         migrate_projects_table(conn)
+        # worker 每轮 poll 都按 status 取任务（见 load_tasks 的
+        # WHERE status IN (...)），无索引时是全表扫描；任务表随
+        # 时间变大后索引是必须的。IF NOT EXISTS 保证老库幂等。
+        conn.execute(
+            "CREATE INDEX IF NOT EXISTS idx_tasks_status"
+            " ON tasks(status)")
         conn.commit()
     finally:
         conn.close()